The technology of frequency convention used for generating a new waveband laser (including second harmonic generation, sum frequency generation, difference frequency etc.) by nonlinear optical crystals is widely used. Thereof one of the key technologies is the nonlinear optical crystal and the design of the nonlinear optical crystals laser second harmonic generator. During the frequency convention it is necessary for the nonlinear optical crystals to satisfy the condition of the conservation of momentum, namely the phase matching condition. The ways of the phase matching mainly comprise angular phase matching, temperature phase matching and quasi phase matching. For example, in the conventional process of second harmonic generation, the angular phasing matching is that the laser with specific polarization direction propagates along one particular direction (phase matching direction) of the nonlinear optical crystal, namely generating second harmonic generation laser. The temperature phase matching is the process that the crystal's refractive indexes is changed via changing the temperature of the nonlinear optical crystal in order to satisfy with the phase matching condition. However, the range of variation for the crystal's refractive index ellipsoid is limited, thus the temperature phase matching can be seen as a finite correction for the angular phase matching. The quasi phase matching is realized in dielectric super-lattices by the modulation of reciprocal lattice to compensate the phase mismatch caused by refractive index dispersion. This method is not practical and rarely used in application. Commonly, the crystal in second harmonic generator should be cut along the phase matching direction and then be manufactured into the required shape. The two optical surfaces need to be performed precision polishing and plated into the different required optical films. The laser is normally or nearly normally incident on the surface of the nonlinear optical crystal for the second harmonic generation.
The nonlinear optical crystals show a layered growth habit. Their modality is the plate-like crystal and they are easy to cleavage along one surface thereon because of their intrinsic structure properties. It is difficult to manufacture these crystals along its phase matching directions. For example, borofluoride beryllium crystal potassium family (the present application comprises two kinds of crystals, namely Potassium Beryllium Fluoroborate KBe2BO3F2, written as KBBF; Rubidium Beryllium Fluoroborate RbBe2BO3F2, written as RBBF). It is easy to cleavage along crystallographic c-surface because of the layered growth habit of the two kinds of crystals, and it is difficult to grow thick along the axis of c direction (namely the optical axis direction). The crystals exhibit flaky form and the two exposed nature face are the crystallographic c-plane, also known as the (001) plane, or the a-b plane. It is difficult to cut along the phase matching direction for second harmonic generation used by KBBF crystal family. To solve the above-mentioned problems, there is provided with a coupler and optical contact prism coupling technology (PCT) with prism-nonlinear optical crystals (ZL 01115313. X; U.S. Pat. No. 6,859,305B2; Japanese Patent 4074124), which successfully solved the problems of the phase matching for KBBF crystal family. This invention, which combined the KBBF crystal being difficult to be cut and the prisms with specific shape, realized the second harmonic generation laser for deep ultraviolet laser (namely wavelength is under 200 nm). This technology also belongs to the angle phase matching.
In the prism coupled device, the KBBF crystal family coupled, with the prisms (typically quartz glass or calcium fluoride crystals) by ordinary optical contact bonding. Since the device is optical coupled between two different materials, the binding force is weak, and the thermal expansion coefficient is different, the inconsistent expandation will be produced when the device is heated, which will cause the separation of the two optical elements and even damaged of the two optical elements interfaces. Especially when KBBF family prism coupled device operating at the medium power laser system and the high power laser system, the laser power is high and the time of the pulse duration is long, which will cause the temperature of the optical interface increase by strong absorption. This usually causes the interface damaged firstly, which will have influence on the laser beam quality and even leading to the separation of the two interfaces, then the whole device is scrapped. Although the diffusion bonded technology is developed (U.S. Pat. No. 8,773,750), which improved the laser damage threshold than the ordinary optical coupled technology. Compared with the photo damage resistance threshold of the KBBF crystal, the value is still too low. For example, for the wavelength of 1064 nm, pulse width of 80 ps and the repetition rate of 1 KHz, the photo damage resistance threshold of KBBF crystal reaching to 900 GW/cm2. For the fundamental frequency optical wavelength of 390 nm, pulse width of 200 fs and repetition rate of 1 KHz, the damage threshold also reaching up to 60 GW/cm2. Under the same condition, the laser damage threshold of the KBBF crystals is nearly one order magnitude larger than that of the BBO crystals. In the currently known nonlinear optical crystals, the KBBF crystals have the highest laser damage threshold. But the laser damage threshold of the KBBF prism coupler is just 300 MW/cm2 under the wavelength of 1064 nm, pulse width of 0.9 ns and repetition rate reaching to 6 KHz.
In addition, another drawback of the prism coupled device is the volume is too much as it coupled with the quartz glass or calcium fluoride crystal prism. As the thermal conductivity for quartz and calcium fluoride is poor, so it is difficult to control the temperature of KBBF in the practical process and maintain the stability of the output power of the laser second harmonic generation.
Currently, the fourth harmonic generation of 266 nm laser output of 1064 nm realized by KBBF crystal is achieved through prism coupling technology. The biggest obstacle for implementing the prism coupling technology is the damage usually happened at the optical interface. Prism coupling technology for deep ultraviolet laser output by KBBF family crystal is only the one technological route. But the prism coupling technology is not the unique way when the fourth harmonic generation laser of 266 nm being output.